U.S. patent number 7,935,645 [Application Number 12/239,028] was granted by the patent office on 2011-05-03 for lightweight high-tensile, high-tear strength biocomponent nonwoven fabrics.
This patent grant is currently assigned to North Carolina State University. Invention is credited to Nataliya V. Fedorova, Behnam Pourdeyhimi, Stephen R. Sharp.
United States Patent |
7,935,645 |
Pourdeyhimi , et
al. |
May 3, 2011 |
Lightweight high-tensile, high-tear strength biocomponent nonwoven
fabrics
Abstract
A method of producing a nonwoven fabric comprising spinning a
set of bicomponent fibers which include an external fiber component
and an internal fiber component. The external fiber enwraps said
internal fiber and has a higher elongation to break value than the
internal fiber and a lower melting temperature than the internal
fiber component. The set of bicomponent fibers are positioned onto
a web and thermally bonded to produce a nonwoven fabric.
Inventors: |
Pourdeyhimi; Behnam (Cary,
NC), Fedorova; Nataliya V. (Kiev, UA), Sharp;
Stephen R. (Holly Springs, NC) |
Assignee: |
North Carolina State University
(Raleigh, NC)
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Family
ID: |
37071173 |
Appl.
No.: |
12/239,028 |
Filed: |
September 26, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090017708 A1 |
Jan 15, 2009 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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11096954 |
Apr 1, 2005 |
7438777 |
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Current U.S.
Class: |
442/361; 442/401;
442/364; 442/329; 442/363; 442/60 |
Current CPC
Class: |
D04H
3/14 (20130101); D04H 13/00 (20130101); D04H
3/16 (20130101); Y10T 442/637 (20150401); Y10T
442/69 (20150401); Y10T 442/64 (20150401); Y10T
156/10 (20150115); Y10T 442/2008 (20150401); Y10T
442/641 (20150401); Y10T 442/681 (20150401); Y10T
442/602 (20150401) |
Current International
Class: |
D04H
1/00 (20060101); D04H 3/00 (20060101); D04H
5/00 (20060101); D04H 13/00 (20060101) |
Field of
Search: |
;442/60,329,361,363,364,401 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 696 629 |
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Feb 1996 |
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EP |
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0 696 691 |
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Feb 1996 |
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EP |
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1 311 085 |
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Mar 1973 |
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GB |
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1 323 296 |
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Jul 1973 |
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GB |
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10-251921 |
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Sep 1998 |
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JP |
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11 131349 |
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May 1999 |
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JP |
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2000 096417 |
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Apr 2000 |
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JP |
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WO 02/44448 |
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Jun 2002 |
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WO |
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WO 2005/004769 |
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Jan 2005 |
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WO |
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Other References
Hedge et al., Bicomponent Fibers, from the website
http://web.utk.edu/.about.mse/pages/Textiles/Bicomponent%20fibers.htm
, Apr. 2004. cited by other .
Chidambaram et al., "Strength Loss in Thermally Bonded
Polypropylene Fibers," Inter Nonwovens Journal, 2000, pp. 27-35,
vol. 9, No. 3. cited by other .
Zhou et al., "New Type Chemical Fiber-Sea-Island Composite
Superfine Fiber," Nonwoven, pp. 41-44, vol. 12, No. 1. cited by
other.
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Primary Examiner: Torres-Velazquez; Norca L
Attorney, Agent or Firm: Womble Carlyle Sandridge &
Rice, PLLC
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is a divisional of U.S. patent application
Ser. No. 11/096,954, filed Apr. 1, 2005, now U.S. Pat. No.
7,438,777 which is incorporated herein by reference in its
entirety.
Claims
What is claimed is:
1. A nonwoven web comprising: a substantially continuous set of
bicomponent fibers, each of said fibers comprising an external
fiber component and one or more internal fiber components; wherein
said external fiber component enwraps said internal fiber
components; said external fiber component having a higher
elongation to break value and a lower melting temperature than said
internal fiber components, wherein said set of bicomponent fibers
comprises a plurality of bond points wherein the external fiber
components of at least two adjoining fibers are completely melted
forming a matrix that encapsulates the internal fiber
components.
2. The nonwoven web of claim 1, wherein said external fiber
component has a melting point of at least twenty degrees Celsius
lower than the melting point of said internal fiber components.
3. The nonwoven web of claim 1, wherein said external fiber
component has a melting point at least one hundred and fifty
degrees Celsius lower than the melting point of said internal fiber
components.
4. The nonwoven web of claim 1, wherein said external fiber
component has an elongation to break value at least one and a half
times greater than the elongation to break value of said internal
fiber components.
5. The nonwoven web of claim 1, wherein said internal fiber
components comprise a copolyetherester elastomer with long chain
ether ester units and short chain ester units joined head to tail
through ester linkages.
6. The nonwoven web of claim 1, wherein said external fiber
component comprises a copolyetherester elastomer with long chain
ether ester units and short chain ester units joined head to tail
through ester linkages.
7. The nonwoven web of claim 1, wherein said internal fiber
components comprise a polymer selected from the group consisting of
nylon 6, nylon 6/6, nylon 6,6/6, nylon 6/10, nylon 6/11, nylon
6/12, polypropylene, and polyethylene.
8. The nonwoven web of claim 1, wherein said external fiber
component comprises a polymer selected from the group consisting of
nylon 6, nylon 6/6, nylon 6,6/6, nylon 6/10, nylon 6/11, nylon
6/12, polypropylene, and polyethylene.
9. The nonwoven web of claim 1, wherein said internal fiber
components comprise a polymer selected form the group consisting of
polyesters, polyamides, thermoplastic copolyetherester elastomers,
polyolefins, polyacrylates, and thermoplastic liquid crystalline
polymers.
10. The nonwoven web of claim 1, wherein said external fiber
component comprises a polymer selected from the group consisting of
polyesters, polyamides, thermoplastic copolyetherester elastomers,
polyolefins, polyacrylates, and thermoplastic liquid crystalline
polymers.
11. The nonwoven web of claim 1, wherein said internal fiber
components comprise a copolymer selected from the group consisting
of styrene-butadiene copolymers, propylene-butene copolymers,
copolymers of ethylene with at least one vinyl monomer, and
copolymers of ethylene with unsaturated aliphatic carboxylic
acids.
12. The nonwoven web of claim 1, wherein said internal fiber
components are multi-lobal or circular.
13. The nonwoven web of claim 1, wherein said internal fiber
components include a plurality of internal fiber components
enwrapped by said external fiber component defining an islands in
the sea bicomponent fiber.
14. The nonwoven web of claim 13, wherein said internal fiber
components include a plurality of internal fiber components which
have different mechanical properties selected from the group
consisting of elasticity, wetness, and flame retardation.
15. The nonwoven web of claim 1 manufactured into a product
selected from the group consisting of a tent, a parachute, an
awning, and a house wrap.
16. A nonwoven fabric comprising: a substantially continuous set of
spunbonded bicomponent fibers, each of said fibers comprising an
external fiber component and one or more internal fiber components;
wherein said external fiber component enwraps said internal fiber
components; said external fiber component having a higher
elongation to break value and a lower melting temperature than said
internal fiber components; wherein said set of bicomponent fibers
comprises a plurality of bond points wherein the external fiber
components of at least two adjoining fibers are completely melted
forming a matrix that encapsulates the internal fiber
components.
17. The nonwoven fabric of claim 16, wherein said external fiber
component has a melting point at least twenty degrees Celsius lower
than the melting point of said internal fiber components.
18. The nonwoven fabric of claim 16, wherein said external fiber
component has a melting point at least one hundred and fifty
degrees Celsius lower than the melting point of said internal fiber
components.
19. The nonwoven fabric of claim 16, wherein said external fiber
component has an elongation to break value at least one and a half
times greater than the elongation to break value of said internal
fiber components.
20. The nonwoven fabric of claim 16, wherein said internal fiber
components comprise a copolyetherester elastomer with long chain
ether ester units and short chain ester units joined head to tail
through ester linkages.
21. The nonwoven fabric of claim 16, wherein said external fiber
component comprises a copolyetherester elastomer with long chain
ether ester units and short chain ester units joined head to tail
through ester linkages.
22. The nonwoven fabric of claim 16, wherein said internal fiber
components comprise a polymer selected from the group consisting of
nylon 6, nylon 6/6, nylon 6,6/6, nylon 6/10, nylon 6/11, nylon
6/12, polypropylene, and polyethylene.
23. The nonwoven fabric of claim 16, wherein said external fiber
component comprises a polymer selected from the group consisting of
nylon 6, nylon 6/6, nylon 6,6/6, nylon 6/10, nylon 6/11, nylon
6/12, polypropylene, and polyethylene.
24. The nonwoven fabric of claim 16, wherein said internal fiber
components comprise a polymer selected form the group consisting of
polyesters, polyamides, thermoplastic copolyetherester elastomers,
polyolefins, polyacrylates, and thermoplastic liquid crystalline
polymers.
25. The nonwoven fabric of claim 16, wherein said external fiber
component comprises a polymer selected from the group consisting of
polyesters, polyamides, thermoplastic copolyetherester elastomers,
polyolefins, polyacrylates, and thermoplastic liquid crystalline
polymers.
26. The nonwoven fabric of claim 16, wherein said internal fiber
components comprise a copolymer selected from the group consisting
of styrene-butadiene copolymers, propylene-butene copolymers,
copolymers of ethylene with at least one vinyl monomer, and
copolymers of ethylene with unsaturated aliphatic carboxylic
acids.
27. The nonwoven fabric of claim 16, wherein said internal fiber
components are multi-lobal or circular.
28. The nonwoven fabric of claim 16, wherein said internal fiber
components include a plurality of internal fiber components
enwrapped by said external fiber component defining an islands in
the sea bicomponent fiber.
29. The nonwoven fabric of claim 28, wherein said internal fiber
components include an plurality of internal fiber components which
have different mechanical properties selected from the group
consisting of elasticity, wetness, and flame retardation.
30. The nonwoven fabric of claim 16, manufactured into a product
selected from the group consisting of a tent, a parachute, an
awning, and a house wrap.
31. The nonwoven fabric of claim 16, wherein the outer surface of
said nonwoven fabric is coated with an impermeable resin layer.
32. The nonwoven fabric of claim 16, wherein said nonwoven fabric
is dyed.
Description
TECHNICAL FIELD
The subject matter disclosed herein relates generally to nonwoven
fabrics used in applications wherein high tensile and high tear
properties are desirable such as outdoor fabrics, house wrap,
tents, awning, parachutes, and the like. More particularly, the
present subject matter relates to methods for manufacturing high
strength, durable nonwoven fabrics and products produced thereof
with high abrasion resistance through the use of bicomponent
spunbonded fibers having different melting temperatures and wherein
the fibers are manipulated such that one component forms a matrix
enveloping a second component.
BACKGROUND
Nonwoven fabrics or webs have a structure of individual fibers or
threads which are interlaid, but not in a regular or identifiable
manner as in a woven fabric. Nonwoven fabrics or webs have been
formed from many processes which include meltblowing, spunbonding
and air laying processes. The basis weight of fabrics is usually
expressed in grams per square meter.
Nonwoven spunbonded fabrics are used in many applications and
account for the majority of products produced or used in North
America. Almost all such applications require a lightweight
disposable fabric. Therefore, most spunbonded fabrics are designed
for single use generally requiring minimum bond strength and are
designed to have adequate properties for the applications for which
they are intended. Spunbonding refers to a process where the
fibers, filaments, are extruded, cooled, and drawn and subsequently
collected on a moving belt to form a fabric. The web thus collected
is not bonded and the filaments must be bonded together thermally,
mechanically or chemically to form a fabric. Thermal bonding is by
far the most efficient and economical means for forming a fabric.
Hydroentangling is not as efficient, but leads to a much more
flexible and normally stronger fabric when compared to thermally
bonded fabrics. Thermal bonding is one of the most widely used
bonding technologies in the nonwovens industry. It is used
extensively in spunbond, meltblown, air-lay, and wet-lay
manufacturing as well as with carded-web formation technologies.
Considerable effort has been spent on trying to optimize the
web-formation processes, bonding processes, and the feed fiber
properties to achieve the desired end-use properties while reducing
the cost of manufacture. One way to reduce the cost of manufacture
is to produce more nonwoven fabric on the same machine by
processing faster. It has been found that satisfactory bonds can be
made faster at higher temperatures, up to a point, after which
satisfactory bonds can no longer be made. This is sometimes
described as "the bonding window closes as the bonding temperature
increases". The processing window at a given process speed is
defined by the maximum and minimum process temperatures that
produce nonwovens with acceptable properties. In other words, it
has been found that as one attempts to process faster, the
difference between the maximum and minimum process temperatures
gets smaller until they merge into a single temperature. At still
higher speeds, no suitable nonwoven can be made, regardless of the
bonding temperature, i.e. the processing window closes.
In addition, over the last 100 years of modern fiber science, it
has been learned that stronger fibers generally make stronger
textile structures when all the other construction factors are the
same. This applies to cords, ropes, knits and wovens. In addition,
for melt-spun fibers, it is possible to make stronger fibers by
increasing fiber orientation and crystallinity, as well as
achieving appropriate fibrillar morphology. This is typically
accomplished by increasing the spinning speed, altering the
quenching conditions, increasing the draw ratio and annealing the
fibers under tension. Hence, it was unexpected when it was found
that thermally point bonded nonwoven fabrics became weaker when
high strength fibers were used and, conversely, yielded stronger
fabrics with appropriate weaker fibers.
Part of the confusion about the strength of nonwovens can be
attributed to the fact that the failure mode changes with bonding
conditions. It has been observed that the strength of the bonded
fabric increases with bonding temperature or with bonding time up
to a point, and then the bonded fabric strength begins to decrease.
For bonding conditions below this peak, failure occurs by bond
disruption, i.e. the bond simply pulls apart. Above the peak,
failure occurs by fiber breakage at the bond periphery. Several
explanations for this latter observation have been provided. One
explanation that has been forwarded is that there is a stress
concentration at the bond periphery, where most failures occur.
Although this is likely to be true, no satisfactory explanation of
the dependence of the stress concentration on bonding conditions
has been provided. Another proposed failure mechanism is that the
fibers are crushed by the calendar rolls and thus weakened at the
bond edge where the edges of the bond point flatten the fibers.
However, as shown by Chidambaram, A., Davis, H., Batra, S. K.,
"Strength Loss in Thermally Bonded Polypropylene Fibers" Inter
Nonwovens J 2000, 9(3) 27 this factor accounted for only a small
portion of the loss of strength. Furthermore, bond strength did not
correlate with bonding pressure, as one would expect for this
failure mechanism. To date, no satisfactory explanation of the
mechanical failure mechanisms of thermally point bonded nonwovens
has been provided.
Thermal bonding can be performed in several ways. In through-air
bonding, a hot fluid, air, is forced through a preformed web. If
the temperature of the fluid is high enough, the fibers may become
tacky and adhere to one another. In this case they form bonds where
two or more fibers come into contact. In infrared bonding,
IR-bonding, infrared light provides the heat. In ultrasonic
bonding, friction between contacting fibers due to the application
of ultrasound causes the fibers to become tacky and bond. In
thermal point bonding, the preformed fiber web is passed between
heated calendar rolls. The rolls may be smooth or embossed with a
bonding pattern. A uniform fabric requires uniform pressure,
uniform temperature and uniform input web. Bonding occurs only
where the fibers contact the heated rolls. Therefore, on a smooth
calendar roll, bonding occurs wherever fibers cross each other
while on an embossed calendar roll, bonding occurs primarily
between the raised areas. This results in bonding "points" or
"spots". In each of these processes, the underlying physics is the
same, the fibers are heated, they form a bond, and they are
subsequently cooled.
Before bonding can occur, a web must be formed. The processes
usually employed include spinning (spunbond), melt-blowing,
wet-laying, air-laying and carding. Each of these produces
different fiber orientation distribution functions (ODF) and web
densities. It is important to recognize that there is an
interaction between the web structure and the efficiency with which
bonds are formed, i.e. bonding efficiency. In the simplest case
where smooth calendar rolls are used, or in through-air bonding,
the maximum level of bonding occurs when the structure is random
since the maximum number of fiber-to-fiber crossovers is achieved.
Thus, the more oriented the structure, the fewer the number of
potential bond sites. The ODF also dictates, to a great extent, the
manner in which the structure undergoes mechanical failure. While
failure can follow different modes, the fabrics tend to fail by
tearing across the preferred fiber direction when the load is
applied parallel to the machine- or cross-directions. At all other
test angles, failure is likely to be dictated by shear along the
preferred direction of fiber orientation.
It is generally observed that the strength of the structure
improves with bonding temperature, reaches a maximum, and then
declines rapidly because of over-bonding and premature failure of
the fibers at the fiber-bond interface. However, regardless of the
bonding temperature, the changes brought about in the web structure
and the microscopic deformations therein are driven by the initial
ODF of the fibers, and therefore are similar for all structures
with the same initial ODF. During load-elongation experiments the
nature of the bonding process controls the point at which the
structure fails, but the behavior up to that point is dictated by
the structure (ODF) and the anisotropy of the bond pattern.
Moreover, the structure stiffness, i.e. tensile modulus, bending
rigidity and shear modulus, continues to increase with bonding
temperature.
After the web is formed, it passes through the calendar rolls where
it is bonded. Thermal point bonding proceeds through three stages:
1) compressing and heating a portion of the web, 2) bonding a
portion of the web, and 3) cooling the bonded web. In calendar
bonding, the bonding pressure appears to have little or no effect
on fabric performance beyond a certain minimum. This is especially
true for thin nonwovens where minimal pressure is required at the
nip to bring about fiber-to-fiber contact. Sufficient pressure is
needed to compact the web so that efficient heat transfer through
conduction can take place. In addition, pressure aids plastic flow
at elevated temperatures, thereby increasing contact area between
the fibers as well as decreasing thickness at the bond even
further. Pressure also aids "wetting" of the surfaces. This
requires fairly minimal pressures. Pressure also constrains the
mobility of the fibers in the bond spot. Over the range of
pressures commercially employed, higher nip pressures do not
necessarily lead to higher performance.
In calendar and through-air bonding, it is quite easy to obtain
under-bonded or over-bonded structures. Under-bonding occurs when
there are an insufficient number of chain ends in the tacky state
at the interface between the two crossing fibers or there is
insufficient time for them to diffuse across the interface to
entangle with chains in the other fiber. The formation of a bond
requires partial melting of the crystals to permit chain relaxation
and diffusion. If, during bonding, the calendar roll temperatures
are too low or if the roll speeds are too high, the polymer in the
mid-plane of the web does not reach a high enough temperature to
release a sufficient number of chains or long enough chain segments
from the crystalline regions. Thus, there will be very few chains
spanning the fiber-fiber interface, the bond itself will be weak,
and the bonds can be easily pulled out or ruptured under load, as
observed.
Over-bonding occurs when many chains have diffused across the
interface and a solid, strong bond has been formed. The fibers
within the bond spot, and at the bond fiber periphery, have lost
their orientation and their strength, but the bond spot itself
represents a more rigid and larger area compared to the fibers
entering the bond spot. However, at the same time, the polymer
chains within the fibers located in the vicinity of the bond, also
relax to lower birefringence as heat diffuses along the fiber
length. Thus the fibers entering the bond have also lost some of
their molecular orientation and consequently their strength at the
fiber-bond interface. The distance that sufficient heat diffuses
along the fiber length subjected to heating depends on the time and
temperature in the nip. It has been observed at high speeds, this
distance should be less than the thickness of the nip, while at
lower speeds the distance should be longer. Since the birefringence
is only reduced where the temperature was high enough to start
melting the crystals, it is only this region that has reduced
strength. Thus the birefringence of the fibers is reduced only in
the region close to the bond periphery and the fibers are weak only
in this region. They may have also become flat and irregular in
shape. The bond site edge becomes a stress concentration point
where the now weaker fibers enter. In a fabric under load, this
mechanical mismatch results in the premature failure of the fibers
at the bond periphery, as observed. Simply put, over-bonding occurs
when too much melting has occurred.
Thermal bonding of nonwoven webs occurs through three steps 1)
heating the fibers in the web, 2) forming a bond through reptation
of the polymer chains across the fiber-fiber interface, 3) cooling
and resolidifying the fibers. In calendar bonding, step 1 must
occur while the web is in the nip. Step 2 must begin while the web
is in the nip to tie the structure together, but it can finish
during the initial portion of step 3. There is excellent agreement
between the required times for heating and forming the bond and
commercial bonding times.
In under-bonded webs, there are too few polymer chains diffusing
across the fiber-fiber interface. During tensile testing, these
bonds simply disintegrate. In well-bonded webs there is sufficient
diffusion of the chains across the interface to form a strong bond,
but only a moderate loss of mechanical properties of the bridging
fibers at the bond periphery. Hence there is an acceptable trade
off between the strength of the bond and the strength of the fibers
at the bond periphery. In over-bonded webs, there is sufficient
diffusion of the chains across the interface to form a strong bond,
but there is a large loss of mechanical properties of the bridging
fibers at the bond periphery. During tensile testing, the fibers
break at the bond periphery.
Hydroentangling results in somewhat different characteristics. The
bonded fibers will be flexible and will have a higher strength than
its calendar bonded counter part. The fabric does not go through
shear failure as easily as thermally point bonded nonwovens.
Bicomponent nonwoven filaments are known in the art generally as
thermoplastic filaments which employ at least two different
polymers combined together in a heterogeneous fashion. Most
commercially available bicomponent fibers are configured in a
sheath/core, side-by-side or eccentric sheath/core arrangement.
Instead of being homogeneously blended, two polymers may, for
instance, be combined in a side-by-side configuration so that a
first side of a filament is composed of a first polymer "A" and a
second side of the filament is composed of a second polymer "B".
Alternatively, the polymers may be combined in a sheath-core
configuration wherein the outer sheath layer of a filament is
composed of first polymer "A" and the inner core is composed of a
second polymer "B".
Bicomponent fibers or filaments offer a combination of desired
properties. For instance, certain resins are strong but not soft
whereas others are soft but not strong. By combining the resins in
a bicomponent filament, a blend of the characteristics may be
achieved. For instance, when the bicomponent fibers are in a
side-by-side arrangement these are usually used as self-bulking
fibers. Self-bulking is created by two polymers within a filament
having a different strain level or shrinkage propensity. Hence,
during quenching or drawing they become crimped. Also, for some
sheath/core configurations, the polymer utilized for the sheath
component may have a lower melting point temperature than the core
component. The outer component sheath component is heated to become
tacky forming bonds with other adjacent fibers.
An additional bicomponent fiber is known as an islands-in-sea
fiber. In such a configuration, a "sea" component forms the sheath,
with the "island" components being the core or cores. Typically,
islands-in-sea fibers are manufactured in order to produce fine
fibers. The production of nanofibers in and of themselves is
infeasible with current technology. Certain fiber size is necessary
to insure controlled manufacturing. Accordingly, to produce
nanofibers, islands-in-sea fibers consist of a sea component which
is soluable and when removed results in the interior fibers being
released. Also, it is known in some circumstances to maintain the
sea component. U.S. Pat. No. 6,465,094 discloses a specific fiber
construction which is of an islands-in-sea type configuration
wherein the sheath, e.g. sea, is maintained to provide the fiber
with distinct properties. Such a structure is akin to a typical
bicomponent sheath/core construction with multi cores enabling
certain fiber properties to be created.
While prior art bicomponent fibers are known, there is a need for a
high strength, lightweight nonwoven fabric.
In view of the aforementioned, it is an object of the present
invention to provide a method for producing high strength
spunbonded nonwoven fabrics;
It is a further object of the present invention to establish a
fiber construction which is bonded in a manner which enables the
fiber to exhibit high tensile and tear strength characteristics
previously unfounded in nonwoven fabrics.
SUMMARY
A method of producing a nonwoven fabric comprising spinning a set
of bicomponent fibers which include an external fiber component and
an internal fiber component. The external fiber enwraps said
internal fiber and has a higher elongation to break value than the
internal fiber and a lower melting temperature than the internal
fiber component. The set of bicomponent fibers are positioned onto
a web and thermally bonded to produce a nonwoven fabric.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is schematic drawing of typical bicomponent spunbonding
process;
FIG. 2 is schematic drawing of typical calendar bonding
process;
FIG. 3 is schematic drawing of typical single drum thru-air bonding
oven;
FIG. 4 is a schematic drawing of a typical drum entangling
process;
FIG. 5 shows cross-sectional view of bicomponent fibers produced
according to the present invention;
FIG. 6 shows a SEM Micrograph of the bonding and the bond fiber
interface of a 108 island nylon/PE spunbonded fabric bonded
thermally;
FIG. 7 shows SEM Micrographs of the bond spot of a 108 island
nylon/PE spunbonded fabric bonded thermally;
FIG. 8 shows SEM Micrographs of the surface of a thru-air bonded
108 island spunbonded fabric;
FIG. 9 shows a magnified portion of the surface of a thru-air
bonded 108 island spunbonded fabric demonstrating fiber to fiber
bonding; and
FIG. 10 shows SEM Micrographs of the surface of a hydroentangled
thru-air bonded 108 island spunbonded fabric.
DETAILED DESCRIPTION
A nonwoven fabric is manufactured utilizing a bicomponent fiber
structure. The bicomponent fiber structure consists of two distinct
fiber compositions which are produced preferably utilizing spun
bound technology with an external fiber component enwrapping a
second internal fiber component. Such construct is known as
sheath/core or islands-in-sea fibers. A sheath/core consists of a
single sheath, external, fiber enwraps a single core, internal,
fiber. In the islands-in-sea construction a single sea, external,
fiber enwraps a plurality of islands, internal, fibers. Examples of
the fibers are shown in FIG. 5. The internal core or islands fiber
component is circumferentially enwrapped by the external sheath or
sea fiber component. With this configuration, the method of the
invention includes the step of forming a single or more layers of
spunbonded filaments wherein the fibers or filaments are
bicomponent with two polymers.
The subject matter disclosed herein relates to methods for
improving the bonding process between respective bicomponent fibers
where the fabric failure is not dictated by the properties of the
fiber-bond interface. In a thermally bonded nonwoven composed of
homocomponent fibers, the fibers lose their properties at the
bond-fiber interface as well as in the bond because of partial
melting of the fibers, as well as potential deformations brought
about locally. The changes in the mechanical properties and due to
high stress concentrations at the fiber bond interface, the
nonwoven tends to fail prematurely.
The inventors have discovered that in a bicomponent fiber in the
form of sheath-core or islands-in-sea, the properties can be
enhanced when the external and internal fiber components are
sufficiently different in their melt properties and the external
fiber is completely melted at a bond point. Additionally, the
bicomponent fibers must have certain differing characteristics. The
sheath or sea component must have a melting temperature which is
lower than the core or island component. This difference should be
at least fifteen degrees Celsius and is preferably twenty degrees
Celsius or more. At the bond point, the external fiber of at least
two adjoining fibers are completely melted forming a matrix which
encapsulates the internal fiber. When the bicomponent fibers
utilized are of the islands-in-sea configuration, the entire sea is
melted and most preferably, the entire sea of two adjoining fibers
is completely melted. Hence, for bicomponent fibers utilizing
islands-in-sea, it is feasible to melt the sea component even in
locations which are not bonded with adjacent fibers.
Additionally, to improve spinnability of said bicomponent fibers,
it is preferred that the thermoplastic materials also have
different viscosity values. Also, the viscosity of the sheath or
sea component must be equal or greater than the core or island
component. Preferably the external fiber has a viscosity of about
one and a half times than that of the internal fiber. Best results
have been obtained when the external fiber has a viscosity of twice
the internal fiber. Such differential in viscosities enables the
matrix to be formed in a manner conducive to forming the high
strength fiber of the invention.
Also, the two components forming the internal and external portions
of the fibers preferably have different elongation to break values.
A suitable measurement of elongation to break values may be
obtained utilizing ASTM standard D5034-95. The internal fiber
preferably has an elongation to break value less than the external
fiber. Preferably, the internal fiber has an elongation to break
value at least thirty percent less than the external fiber. For
instance the external fiber may have an elongation to break value
of fifty percent and the internal fiber has an elongation to break
value of thirty percent. This difference facilitates in the shear
and tensile forces applied to the nonwoven fabric to be transferred
to the internal (stronger) fiber through the matrix (weaker)
thereby enhancing the bond strength of the fibers.
While the invention can be maintained by forming a matrix, with
additional strength being obtained with either the viscosity of the
fibers being different or the elongation to break of the fibers
being different, best results have been obtained by forming a
matrix with an internal fiber being more viscous than the external
fiber and the internal fiber having a lower elongation to break
value.
FIG. 1 illustrates the typical spunbond process. In a spunbonded
process, small diameter fibers are formed by extruding molten
thermoplastic material as filaments from a plurality of fine
capillaries of a spinneret having a circular or other
configuration, with the diameter of the extruded filaments then
being rapidly reduced. As shown in FIG. 1, a first component
thermoplastic is positioned in a first polymer hopper and a second
component thermoplastic is positioned in a second polymer hopper.
The components are then pumped through a spin pack and joined
together to form a conjugate fiber. This conjugate fiber is
quenched and attenuated and positioned onto a forming belt. The
fiber is then bonded.
In the preferred embodiment, the external fiber component
thermoplastic is utilized to form an external sheath or sea for the
fiber and the internal fiber component thermoplastic is utilized to
form the internal core or islands. Examples of polymer components
desired to be utilized for the sea are polyethylenes, linear low
density polyethylenes in which the alpha-olefin comonomer content
is more than about 10% by weight, copolymers of ethylene with at
least one vinyl monomer, copolymers of ethylene with unsaturated
aliphatic carboxylic acids.
Additionally, for the sea component and/or island component other
preferred thermoplastics include those wherein the polymers are
selected from the group of thermoplastic polymers wherein said
thermoplastic polymer is selected from nylon 6, nylon 6/6, nylon
6,6/6, nylon 6/10, nylon 6/11, nylon 6/12 polypropylene or
polyethylene. Additionally, other suitable thermoplastics include
those wherein the thermoplastic polymer is selected from the group
consisting of: polyesters, polyamides, thermoplastic
copolyetherester elastomers, polyolefines, polyacrylates, and
thermoplastic liquid crystalline polymers. Preferably, the
thermoplastics include those wherein the polymers are selected from
the group of thermoplastic polymers comprising a copolyetherester
elastomer with long chain ether ester units and short chain ester
units joined head to tail through ester linkages. More preferably,
the polymers for the core, the islands, the sheath or the sea are
selected from the group of thermoplastic polymers fabricated in a
temperature range of 50.degree. C. to 450.degree. C.
The shape of the core or islands filaments may be circular or
multi-lobal. Furthermore, when the bicomponent fiber is of an
islands-in-sea configuration, the islands may consist of fibers of
different materials. For instance, certain polymers may be
incorporated to contribute to wettability of the nonwoven web.
These thermoplastics may include without limitation polyamids,
polyvinyl acetates, saponified polyvinyl acetates, saponified
ethylene vinyl acetates, and other hydrophilic materials. Polymers
are generally considered to contribute to a nonwoven fabrics
wettability if a droplet of water is positioned on a nonwoven web
made from the conjugate filaments containing the respective
polymeric components and has a contact angle which is a) less than
90 degrees measured using ASTM D724-89, and b) less than the
contact angle of a similar nonwoven web made from similar filaments
not containing the wettable thermoplastic.
Additionally, polymers may be included which contribute elastic
properties to the thermoplastic nonwoven web. Such polymers include
without limitation styrene-butadiene copolymers; elastomeric
(single-site, e.g. metallocene-catalyzed) polypropylene,
polyethylene, and other metallocene-catalyzed alpha-olefin
homopolymers and copolymers having densities less than about 0.89
grams/cc; other amorphous poly alpha-olefins having density less
than about 0.89 grams/cc; ethylene vinyl acetate, copolymers;
ethylene propylene rubbers; and propylene-butene-1 copolymers and
terpolymers.
Once the multicomponent fiber has been spunbond, it is placed onto
a belt to create substantially continuous filaments of fibers. A
substantially continuous filament of fibers refers to filaments or
fibers prepared by extrusion from a spinneret, which are not cut
from their original length prior to being formed into a nonwoven
web or fabric. Substantially continuous filaments or fibers may
have average lengths ranging from greater than about 15 cvm to more
than one meter, and up to the length of the nonwoven web or fabric
being formed. The definition of "substantially continuous filaments
or fibers" includes those which are not cut prior to being formed
into a nonwoven web or fabric, but which are later cut when the
nonwoven web or fabric is cut. The substantially continuous
filament of fibers form a nonwoven web on the belt and are bonded
to create a nonwoven fabric.
Depending on the ultimate utilization of the nonwoven fabric, the
substantially continuous fibers may be subjected to varying
processes. If the highest strength nonwoven fabric is desired, the
fibers will be subjected to thermal bonding via a smooth calendar.
Alternately, the fabric may be subject to thermal bonding via point
bonding. If a more flexible nonwoven fabric of high strength is
desired, the fibers may be subjected to thermal bonding via thru
air. For the thermal bonding process, the temperature of the fabric
does not exceed the melting point of the sea or sheath by more than
the difference than the melting point of the islands or core. For
instance, in the preferred embodiment, the external component has a
melting temperature which is twenty to a hundred and fifty degrees
Celsius lower than the melting temperature of the internal fiber.
Consequently, the fabric surface temperature would not exceed the
melting point of the external fiber by more than twenty degrees in
the first instance or a hundred and fifty degrees in the second
instance. FIG. 2 is a schematic of a typical calendar bonding
process. FIG. 3 illustrates a typical single drum thru-air bonding
oven.
If even a more flexible fabric of high strength is desired, the
fibers may first be subjected to hydroentangling prior to being
thermally bonded either via thru hot air or a smooth calendar.
However, the inventors have discovered that in fabrics that are
about 5 ounces per square yard or heavier, hydroentangled webs can
lose their properties because of de-lamination at hydroentangling
pressures of up to 250 bars. Therefore, for larger structures, a
combined process where the structure needle punched, is
hydroentangled and is subsequently thermally bonded, may be
preferable. In one configuration the nonwoven fabric is exposed to
the hydroentanglement process. In another, only one surface of the
fabric is exposed to the hydroentanglement process. For the
hydroentanglement process, the water pressure of corresponding
manifolds preferably is between ten bars and one thousand bars.
FIG. 4 illustrates a typical drum entangling process.
Additionally, the surface of the nonwoven fabric may be coated with
a resin to form an impermeable material. Also, the resultant fabric
may be post-processed after bonding with a dye process.
As described in the background, a nonwoven fabric may fail due to
either shear forces or tensile forces rupturing the fibers
themselves or the fiber bonds. Applicants' have discovered a
bonding process which enables a multi-component nonwoven fabric to
exhibit strength at least four times greater than similarly bonded
monofilament fabrics.
The thermal bonding mechanism is one where the lower melting point
sea or sheath melts and protects the islands or the core.
Consequently, there is little or no damage to the islands and the
sea acts as a binder or a matrix holding the structure together
transferring the stress to the stronger core fibers. FIGS. 6-10
shown scanning electron microscope images of bond interfaces of a
hundred and eight islands-in-sea bicomponent fiber consisting of
nylon islands enwrapped by a polyethylene sea. As shown by these
images, the fibrous structures of the islands are preserved. This
will be expected to result in higher tensile properties. Similarly,
when the tear propagates through the fabric, the islands will be
released, bunch together and help absorb energy resulting in high
tear properties.
Tests have shown that the invention results in a calendered
nonwoven fiber having a tongue tear strength four times greater in
the machine direction and twice as great in the cross direction
than a similarly bonded homogeneous nylon fiber and a grab tensile
strength one and a half times greater in the machine direction and
almost four times as great in the cross direction.
EXAMPLES
Several examples are given below demonstrating the properties of
the fabrics produced.
All fabrics weighed about 180 g/m.sup.2.
Example 1
100% Nylon Hydroentangled Samples at Two Energy Levels
TABLE-US-00001 Specific Calender Energy Temperature MD Standard CD
Standard Bonding [kJ/kg] [C.] Mean Error Mean Error 100% Nylon -
Tongue Tear [lb] Calendered Only 0 200 11.90 1.99 11.04 0.79
Hydroentangled Only 6568.72 0 16.00 1.31 15.73 2.22 Hydroentangled
and Calendered 6568.72 200 9.00 0.69 14.46 0.63 100% Nylon - Grab
Tensile [lb] Calendered Only 0 200 100.31 4.68 73.92 6.88
Hydroentangled Only 6568.72 0 170.34 5.17 92.58 5.35 Hydroentangled
and Calendered 6568.72 200 157.60 6.84 81.37 6.40
Note that for a monofilament, hydroentangled sample appears to have
the highest performance. This may be expected because the
mechanical bonds do not necessarily influence the fiber's
integrity, wherein the thermal bonds create weak spots in the fiber
resulting in a weaker structure.
Example 2
75/25% Nylon Islands/PE Sea. 108 Islands
TABLE-US-00002 Specific Calender Energy Temperature MD Standard CD
Standard Bonding [kJ/kg] [C.] Mean Error Mean Error 75/25%
Nylon/PE, 108 islands - Tongue Tear [lb] Calendered Only 0 145
39.44 3.11 40.22 3.13 Hydroentangled Only 6568.72 0 16.00 1.31
15.73 2.22 Hydroentangled and Calendered 6568.72 145 38.16 2.98
28.45 0.58 75/25% Nylon/PE, 108 islands - Grab Tensile [lb]
Calendered Only 0 145 322.63 17.03 175.27 6.78 Hydroentangled Only
6568.72 0 59.32 1.83 96.94 2.35 Hydroentangled and Calendered
6568.72 145 231.15 8.70 128.15 17.29
Note that the Calendered only appears to be the best in the case of
bicomponent fibers and the hydroentangled only sample has the
lowest performance.
Example 3
75/25% Nylon Islands/PE Sea, Calendar Bonded with Varying Number of
Islands. 0 Islands Refers to 100% Nylon Samples Produced at their
Optimal Calendar Temperature
TABLE-US-00003 Stan- Stan- No. of MD dard CD dard Islands Mean
Error Mean Error Tongue Tear [lb] - Calender Bonded at 145 C. 0
11.9 1.99 11.04 0.79 1 28.05 1.03 34.84 1.32 18 34.95 0.55 27.29
0.73 108 39.44 3.11 40.22 3.13 Grab Tensile [lb] - Calender Bonded
at 145 C. 0 100.31 4.68 73.92 6.88 1 415.50 17.98 242.15 8.19 18
425.94 6.42 256.68 13.79 108 322.63 17.03 175.27 6.78
Note that all islands-in-sea samples are significantly superior to
the 100% nylon. The islands only account for 75% of the total fiber
mass and are improved by a factor of 4 or more with simple calendar
bonding.
Articles which may be manufactured utilizing the high strength
bicomponent nonwoven include tents, parachutes, outdoor fabrics,
house wrap, awning, and the like.
* * * * *
References